Starter-drive

ABSTRACT

A starter-drive for use between an aircraft engine and a generator to transmit power in either direction between the engine and the generator, for driving the engine from the generator in a starting mode, and for driving the generator from the engine in a generating mode, including, a generator shaft, an engine shaft, a differential for transmitting power from the engine shaft to the generator shaft, a hydrostatic transmission including one hydraulic unit connected for rotation with the generator shaft, and a second hydraulic unit connected for rotation with a control gear in the differential, a first one-way clutch connecting the second hydraulic unit to drive the engine shaft exclusively through the hydrostatic transmission during starting, a second one-way clutch connecting the engine shaft to the differential to transmit power from the engine to the generator after the engine is started, and means for varying the displacement of one of the hydraulic units to bring the engine shaft up to speed during the starting mode and to add or subtract speed in the differential during the generating mode.

United States Patent [191 Aleem 1 Jan. 22, 1974 STARTER-DRIVE [75]Inventor: Mohammed A. Aleeni, Rockford, Ill.

[73] Assignee: Sundstrand Corporation, Rockford,

Ill.

[22] Filed: Sept. 11, 1972 [21] App]. No.: 288,153

Primary ExaminerAl1an D. Herrmann Attorney, Agent, or Firm-Axel A.Hofgren et a1.

[ ABSTRACT A starter-drive for use between an aircraft engine and agenerator to transmit power in either direction between the engine andthe generator, for driving the engine from the generator in a startingmode, and for driving the generator from the engine in a generatingmode, including, a generator shaft, an engine shaft, a differential fortransmitting power from the engine shaft to the generator shaft, ahydrostatic transmission including one hydraulic unit connected forrotation with the generator shaft, and a second hydraulic unit connectedfor rotation with a control gear in the differential, a first one-wayclutch connecting the second hydraulic unit to drive the engine shaftexclusively through the hydrostatic transmission during starting, asecond one-way clutch connecting the engine shaft to the differential totransmit power from the engine to the generator after the engine isstarted, and means for varying the displacement of one of the hydraulicunits to bring the engine shaft up to speed during the starting mode andto add or subtract speed in the differential during the generating mode.

9 Claims, 4 Drawing Figures 7/2000 RPM STARTER-DRIVE BACKGROUND OF THEINVENTION the generator shaft is connected to a variable displacementhydraulic unit in the hydrostatic transmission and to one element in thedifferential. A second gear element in the differential is connected tothe engine shaft, and the second hydraulic unit in the hydrostatictransmission is connected to the control gear in the differential. Suchsystem operates in a highly desirable fashion in that the generator maybe accelerated to its synchronous speed with a minimum load before it isutilized as an induction motor to start the engine. Thereupon, theengine is started with a minimum of losses and ultimately the system hasan acceptable efficiency when operating at a constant speed transmissionduring the generating mode. However, both hydraulic units in thehydrostatic transmission are of variable displacement construction forthe reason that both are rotated while the generator is accelerated tosynchronous speed preparatory to starting and the displacement of theunits should be adjusted to zero in order to reduce the load on thegenerator while accelerating. Additionally, the system requiresrelatively large hydraulic units to obtain the desired starting modebecause the hydrostatic transmission operates through the differential.

Prior British Pat. No. 1,199,145 relates to a starterdrive systemutilizing a mechanical gear differential in association with a Perburytransmission in an arrangement wherein the starting mode is obtained byoperating the generator as a motor to supply starting power to theengine through the Perbury transmission and the mechanical-differential.During the generating mode, power is transmitted from the engine to thegenerator through'the Perbury transmission only. In the starting mode,power is transmitted from the differential to the engine through aone-way clutch, and in the generating mode, power is transmitted fromthe engine to the Perbury transmission through a second one-way clutchwhich bypasses the differential. While the Perbury transmission isreversible, it has the disadvantage that it is difficult to obtain zerooutput with the input rotating, because of the difficulty of adjustingthe rollers in the transmission to that extent. Further, in thearrangement described, thegears in the differential, though nominallyunloaded during the generating mode, are actively rotating, as aresultof which there is undesirable wear while the gearing is notutilized.

SUMMARY OF THE PRESENT-INVENTION It is a general object of the presentinvention to provide a new and improvedstarter-drive utilizing amechanical gear differential and a hydrostatic transmission between thegenerator shaft and the engine shaft.

According to the present invention, the hydrostatic transmissionincludes a variable displacement unit adjacent the generator shaft and afixed displacement unit adjacent the engine shaft. The differentialincludes a first gear rotating with the generator shaft, a control gearrotating with the fixed displacement hydraulic unit, and a third gearrotatable with the engine shaft. A first one-way clutch is utilizedbetween the fixed displacement hydraulic unit and the engine shaft tobypass the differential during the starting mode. During the generatingmode, a second one-way clutch is utilized between the engine shaft andthe differential for transmitting most of the power from the engine tothe generator through the mechanical gearing, while the hydrostatictransmission is utilized for adding or subtracting speed in thedifferential to maintain a constant output.

The preferred arrangement as described allows for unloading thegenerator while it is brought up to speed preparatory to the startingmode because the variable hydraulic unit adjacent the generator shaftmay be placed at zero displacement. Further, the hydrostatictransmission has low losses during the starting mode and the size of thehydraulic units may be reduced substantially. Additionally, thearrangement provides for acceptable efficiency during the generatingmode when most of the power is transmitted through the mechanicaldifferential.

Another object is to provide a new and improved starter-drive of thetype described including a first conduit connecting the hydraulic unitsfor supplying high pressure fluid from the first hydraulic unit to thesecond hydraulic unit during the starting mode, a second conduitconnecting the hydraulic units for supplying high pressure fluid fromthe second hydraulic unit to the first hydraulic unit during thegenerating mode, a control valve responsive to pressure in the firstconduit and controlling the, variable displacement hydraulic unit tolimit the pressure in the first conduit, and means for establishing ahigh pressure setting on the control valve during the starting mode anda low pressure setting during the generating mode.

It is also an object of the invention to provide a new and improvedstarter-drive of the character mentioned including a governor valveresponsive to speed of the generator shaft and controlling thedisplacement varying means during the generating mode to provideconstant generator speed regardless of variation in the speed of theengine, together with selectively operable bias means controlling thegovernor valve during the starting mode to vary the displacement of thevariable hydraulic unit to bring the engine shaft up to speed.

Other objects and advantages will become apparent from the followingdescription taken in connection with the accompanying drawings.

FIG. 1 is a diagrammatic illustration of the mechanical components of astarter-drive embodying the principles of the present invention;

FIG. 2 is a graph illustrating the speed and direction of rotation ofthe fixed hydraulic unit in the transmission illustrated in FIG. 1;

FIG. 3 is a graph illustrating the speed of rotation of the input andcontrol gearing in the differential; and

FIG. 4 is a hydraulic circuit diagram of the starterdrive.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT According to the presentinvention, use is made of an installed alternating current generator inan aircraft as Once the engine is up to idle speed, the drive functionsfor transmitting power from the engine to the generator to drive thelatter at a constant speed regardless of variations in engine speed andload.

The transmission provides a high degree of flexibility required fortransmitting controlled torque in either direction between an engine anda generator. It provides for engine starting from a 400 Hz 1 15 voltsupply, and constant speed generator drive for 400 Hz generating systemof 90-135 KVA suitable for use in all major airborne constant frequencygenerator systems for military and commercial aircraft.

The functional arrangement of the various elements is illustrateddiagrammatically in FIG. 1. As illustrated there, a shaft 10 is adaptedto be connected with an engine and a shaft 11 is adapted to be connectedwith a generator. The two shafts are interconnected by a drive embodyingthe principles of the present invention including a hydrostatictransmission 12, a mechanical differential 14, an overrunning clutch 16which drives the engine during the starting mode and an overrunningclutch 18 which drives the differential during the generating mode. Thehydrostatic transmission 12 includes a variable displacement hydraulicunit 19 and a fixed displacement hydraulic unit 20 interconnected inclosed hydraulic circuit. During the generating mode, when the generatoris driven by the engine, power is supplied from the engine through theone-way clutch l8 and the differential 14 while the hydrostatictransmission is utilized to add or subtract speed in the differentialfor purposes of maintaining constantoutput speed at the shaft 11. Duringthe starting mode, the generator is utilized to drive the engine throughthe hydrostatic transmission 12 and the one-way clutch 16.

As illustrated diagrammatically in FIG. 1, the differential 14 includesa central carrier 22 including a pair of pinions 23 and 24 rotatablethereon and meshing with each other. Additionally, the pinion 23 engagesa ring gear 26, and the pinion 24 engages a ring gear 27. During thegenerating mode, the power train from the input shaft 10 to the outputshaft 11 includes a gear G1 on the input shaft meshing with a gear G2which drives one-way clutch 18 in turn driving the carrier 22 in thedifferential. Assuming the ring gear 26 is held stationary, rotation ofthe carrier 22 causes rotation of the ring gear 27 which has an integralpinion G6 driving a gear G7 on the generator shaft 11.

In order to maintain constant output speed at the generator shaft 11 inspite of variations in the input speed at the engine shaft 10, thehydrostatic transmission 12 is utilized to add or subtract speed in thedifferential as may be necessary. To this end, the gear G7 on thegenerator shaft meshes with a gear G8 rotatable with the variablehydraulic unit 19. A gear G rotatable with the fixed hydraulic unit 20meshes with a gear G3 in turn meshing with a gear G4 integral with thering gear 26 in the differential. Gear G3 drives the one-way clutch 16on the engine shaft for purposes of transmitting power to the engineduring the starting mode.

In the starting mode, when the generator is operated as an electricmotor, power is transmitted from the generator shaft 11 to the variablehydraulic unit 19 which functions as a pump supplying fluid underpressure to the fixed hydraulic unit 20 which functions as a motor todrive the engine shaft 10 through the one-way clutch 16 while thecarrier 22 overruns the gear G2 by virtue of the clutch 18. Thus in thismode of operation, the power is transmitted entirely through thehydrostatic transmission. During the starting mode, the generator shaft11 is driven at substantially constant speed and the engine shaft 10 isaccelerated by placing the variable hydraulic unit 19 into stroke.

During the generating mode, the engine shaft 10 drives the generatorshaft 11 through the one-way clutch l8 and the differential l4, and theinput shaft 10 overruns the gear G3 by virtue of the clutch 16. Thevariable hydraulic unit is driven at constant speed by the gearing G7,G8, and the fixed hydraulic unit 20 is connected by gears G5 and G3 torotate with gear G4 and ring gear 26 which function asthe control gearin the differential. During the initial phases of constant speedgenerator drive, when the input speed at the engine shaft is relativelylow, the variable unit 19 functions as a pump and the fixed unit 20functions as a motor which adds speed to the differential. At high speedof the engine shaft 10, the fixed unit 20 functions as a pump in amanner to subtract speed from the differential.

The operation of the drive is illustrated in the graphs of FIGS. 2 and3. Referring to FIG. 2, note that on initiation of the starting mode,the fixed unit is stationary. As the variable unit is put into stroke,the fixed unit rotates and power is transmitted to the engine shaft 10at increasing speed as illustrated by the line 30 in FIG. 2. When theengine attains a speed sufficient to sustain itself, the starting modeis concluded, as indicated at the point 32 in FIG. 2, at which time thedisplacement of the variable unit is reversed from full displacement inone direction to full displacement in the opposite direction, and thisresults in rotating the fixed unit at maximum speed in a negativedirection as indicated at the point 34 in FIG. 2, which corresponds toengine idle speed. In the constant speed generating mode, when theengine speed is in the low range, the variable unit 19 functions as apump and the fixed unit 20 adds speed to the differential as representedby the lower portion of the line 36 in FIG. 2. As the engine speedincreases, the displacement of the variable unit is reduced, and thespeed of the fixed unit is gradually reduced to zero, at which time thecontrol gear 26 is held stationary in the differential and the drive isentirely mechanical from the engine shaft 10 to the generator shaft 11.When the speed of the engine shaft exceeds that necessary to provide thedesired output speed, the fixed unit is driven by the differential tosubtract speed from the differential, as represented by the upper halfof the line 36 in FIG. 2.

Referring to FIG. 3, the acceleration of the gear G2 is represented byline 37. The gear G2 rotates proportional to rotation of the engineshaft 10 so that the line 37 also indicates the gradual straight-lineacceleration of the engine shaft 10 from the beginning of the startingmode through the generating mode. During the starting mode, the carrier22 overruns the gear G2 as illustrated by the line 38. However, in thetransition from the starting mode to the generating mode, the speed ofthe carrier drops substantially as represented by the line 39, whereuponit is picked up by the clutch 18 and thereafter rotates with the gear G2during the generating mode, as illustrated by the upper portion of line37. While the exact speeds of the gears G3 and G4 are not illustrated inFIGS. 2 and 3, it will be understood that the speed of such gears variesproportionally with the speed of the fixed unit as illustrated in FIG.2.

Referring now to FI GQ4, it will be mired that the hydraulic units 19and 20 are preferably of an axial piston type. The fixed displacementunit 20 includes a rotatable cylinder block 40 having an annular seriesof axially disposed cylinders with reciprocable pistons 42. The pistonshave spherical ends engaged with bearing shoes reacting on a stationarycam or swashplate 44. The cylinder block 40 is splined on a shaft 46carrying the gear G5.

T he v ariable displacrnnt hydraulic unit T9 includes a rotatablecylinder block 50 having an annular series of axially disposed cylinderswith reciprocable pistons 52. The pistons have spherical ends carryingbearing shoes reacting against a pivotally mounted swashplate 54 whichis movable in opposite directions from the neutral central position tovary the displacement to maximum values in opposite directions fromzero. The cylinder block 50 is splined on a rotatable shaft 56 carryingthe gear G8.

Displacement of the variable unit 19 is controlled by a displacementvarying means including a control cylinder 60 having a reciprocablepiston 62 connected by a rod 63 to the variable angle swashplate 54. Acoiled compression spring 64 in the cylinder urges the swashplate 54toward a maximum displacement position in one direction. Fluid underpressure may be supplied to the end of the control cylinder 60 through aport 65 for moving the swashplate against the bias of the spring 64.

Control fluid is supplied to the control cylinder 60 under control of areverse torque valve 68 or a speed responsive governor 69. A charge pumpis provided at 70 for supplying control fluid under pressure to thereverse torque valve 68 and the governor valve 69. The charge pump 70delivers control fluid under pressure to a conduit 72 which leads to thereverse torque valve 68. The conduit 72 communicates with the governorvalve 69 through a passage 73.

The variable displacement unit 19 and the fixed displacement unit 20 areinterconnected in a closed hydraulic circuit by means of a first conduit75 and a second conduit 76. The conduit 75 supplies fluid under pressurefrom the variable unit 19 to the fixed unit 20 during the starting mode.The conduit 76 conducts high pressure between the hydraulic units duringoperation in the generating mode. The charge pump 70 supplies makeupfluid to the circuit connecting the units 19 and 20 by means of aconduit 78 which leads to a pair of spring-biased check valves 80 and 81leading respectively to the conduits 75 and 76. It will be understoodthat high pressure in one of the conduits 75 and 76 will maintain theassociated check valve closed, while low pressure in the other conduitwill permit the associated check valve to be opened for admission ofmakeup fluid from the charge pump 70. The pressure of charge fluid iscontrolled by an appropriate relief valve 82 communicating with thecharge pump outlet.

The reverse torque valve 68 includes a charge port 86 communicating withthe charge conduit 72, a drain port 87, and a control port 88communicating with a conduit 89 leading to the control cylinder 60.Communication between the supply port 86 and the control port 88 iscontrolled by a reciprocable valve member 90 having a reduced stemportion 92. The valve member 90 is biased by a spring 93 to the positionillustrated, in which a valve stem portion 95 blocks communicationbetween the ports 86 and 88. The spring 93 is seated in a cup-shapedpiston-like member 97 which is I adapted to be adjusted for purposes ofvarying the compression of the spring 93. In operation, the torque valve68 senses the pressure of fluid in conduit through the medium of apassage 99 leading to the upper end of the valve member 90.

In order to vary the setting on the spring 93, the spring seat 97 isadjustable in the valve housing by means of fluid under pressure actingagainst the lower end of the valve seat. In particular, control fluidunder pressure may be supplied to the lower end of the valve seat 97through a passage 102 leading from a motoring solenoid valve 104. Whenfluid is supplied to the lower end of the spring seat 97 through passage102, the spring seat is movable upwardly to a position where the upperend contacts a stop 105. At that time, the spring is set at the highestcompression.

The spring seat 97 abuts a piston 106, and fluid may be supplied to thelower end of the piston 106 through a passage 108 leading from themotoring solenoid valve 104. When fluid is supplied to the lower end ofpiston 106, it is movable upwardly in the valve housing to a positionwhere it engages a stop 109. Such movement has the effect of moving thespring seat to an intermediate position where the compression of thespring 93 is less than maximum and more than the minimum compressionwhich exists when the parts are positioned as illustrated in FIG. 4.

In operation, the high compression setting is placed on the spring 93during the starting mode, and the low compression setting is utilizedduring the generating mode. If it is desired to turn the engine overwithout starting, the intermediate spring compression may be utilizedpltwill be understood that the torque valve member senses the pressure inthe conduit 75, and when the pressure exceeds the setting of the spring,the valve member 90 is movable to communicate the charge fluid passage72 with the control passage 89 to supply fluid under pressure to thecylinder 60 for adjustment of the swashplate in a manner to reduce thepressure in the conduit 75. At the same time, excess pressure in conduit75 is relieved through drain port 87.

The setting of the spring 93 is controlled by the motoring solenoidvalve and a start solenoid valve 112. The motoring solenoid valveincludes a port 106 for re ceiving charge fluid from the start solenoidvalve 112. Fluid from the port 106 is supplied selectively to thepassage 102 or passage 108 by a valve member 114 with a reduced stemportion 116. The valve member is normally positioned as illustrated tocommunicate the port 106 with the passage 102, so that when the startsolenoid valve 112 is energized, the spring seat 97 is moved to the highcompression position. When the motoring solenoid valve is energized, thereduced stem portion 116 is moved downwardly to a place where itcommunicates the port 106 with the passage 108 to move the spring seatto the intennediate position.

The start solenoid valve 112 includes a port 120 for receiving chargefluid from the conduit 72, and a port 121 communicating with themotoring solenoid valve 104. A valve stem in the valve 112 is normallypositioned to block communication between the ports 120 and 121. Uponenergization of the valve 112, the stem is moved to the positionillustrated where the ports 120 and 121 communicate to deliver chargefluid to the motoring valve 104. Thus, when the start solenoid valve isenergized alone, a high pressure setting is placed on the torque valve68. When both the motoring valve 104 and the start valve 112 areenergized, the intermediate setting is placed on the torque valve 68.When neither valve is energized, the low pressure setting is utilized inthe valve 68.

The governor valve mechanism 69 includes a rotatable head 130 withpivoted flyweights 132 having crank arms adapted to control the axialposition of a valve stem 134 which is normally biased toward the left asviewed in FIG. 4 by a spring 135. The rotatable head 130 is driven by agear 137 thereon which is rotatable with the generator shaft 11, so thatthe governor is responsive to the speed of the generator shaft. Thevalve stem 134 includes an annular groove 138 which communicates with apassage 140 leading from the governor to the control cylinder 60. Whenthe valve stem 134 is moved to the left as viewed in FIG. 4, the controlgroove 138 is placed in communication with a charge pressure port 142 tosupply control fluid to the control cylinder 60. When the valve stem 134is moved toward the right in FIG. 4, the control groove 138 places thecontrol cylinder 60 in communication with a drain passage 144 so thatfluid is exhausted from the control cylinder 60.

The governor mechanism includes a magnetic trim control in the form ofan electrical coil 146 positioned adjacent the flyweights 132 in amanner to alter the effect of centrifugal force so as to permitimposition of an artificial control on the valve stem 134 responsive tovariation of current in the coil. In the starting mode, while thegenerator shaft is rotating at a constant speed as the inputshaft, thetrim coil 146 is utilized to program the movement of the valve stem 134controlling the cylinder 60 in a: manner to increase displacement of thevariable unit from neutral toward the position illustratedin FIG. 4 forpurposes of bringing the engine shaft up to a speed at which, the enginecan become self-sustaining.

In operation of the system as described above, the starting sequence isinitiated by a switch available to the pilot in an aircraft cockpit. Thegenerator rotating field is shorted, establishing the generator as aninduction motor. The generator accelerates as an induction motor to apredetermined speed. At the predetermined speed, the generator controlinitiates field control of the generator to provide synchronous motoroperation. When the synchronous motor condition is attained, a signal isprovided to the start solenoid valve to initiate engine cranking.

As the generator initiates rotation of the generator shaft 11, thecharge pump 70 driven thereby develops pressure in the conduits 72 and78 which is relieved across the reverse torque valve 68 and the governorvalve 69. In this interim, the variable displacement hydraulic unit 19is held at zero displacement.

When the signal is provided to energize the start solenoid valve 112, asignal is also provided in the generator control for programming themagnetic trim device 146 in a manner to move the variable unit intostroke in the direction illustrated in FIG. 4. As the rotating unit 19is placed into stroke, it provides fluid flow and pressure to the fixeddisplacement hydraulic unit to initiate rotation of the latter and theengine shaft. The pressure of working fluid in the conduit 75 during thestarting operation is controlled by the reverse torque valve 68.Energization of the start solenoid valve 112 supplies fluid underpressure to the reverse torque valve to establish the high pressuresetting on the valve. In the event the setting of the valve 68 isexceeded by the pressure of fluid in the conduit 75, the valve stem 90is moved downwardly as viewed in FIG. 4 to a position for supplyingfluid under pressure from the charge conduit 72 to the control cylinder60 to reduce the displacement and thereby control the pressure.

When the engine reaches a self-sustaining speed which permits startercut-off, the speed is sensed and the power to the generator isdisconnected so that the generator stops accelerating the engine. Themagnetic trim is discontinued in the governor unit, and the tendency ofthe generator shaft to reduce speed drives the governor to a positionwhich supplies fluid under pressure to the control cylinder 60 to movethe swashplate 54 overcenter to reverse the rotation of the hydraulicunit 20 as illustrated in FIG. 2. While the speed of the generator shaftremains below a desired output speed, such as 12,000 rpm, for example,the governor valve ports the control cylinder 60 to the drain conduit144 and the swashplate 54 moves back toward the position illustrated inFIG. 4 in an effort to bring the speed of the generator shaft up to thedesired value.

When the starting mode is discontinued and the generating mode isinitiated, the start solenoid valve 112 is deenergized and the torquevalve 68 reverts to the low pressure setting. During the generatingmode, the conduit 76 is normally at high pressure and the conduit isnormally at low pressure. However, in the event that the generatorshould start motoring, and pressure begins to rise in the conduit 75,the reverse torque valve isadapted to respond at the low pressuresettingfor purposes of reducing the displacement in the variable unit 19.

In the generating mode, when the speed of the engine shaft is less thanthat required to produce the desired 'speed of the generator shaft 11,the hydrostatic transmission drives the control gear G4 in a reversedirection to add speed in the'differential. After the straightthroughdrive is attained, where the engine shaft 10 rotates at a speed to drivethe generator shaft 11 at the proper speed with the control gear G4stationary, the variable unit swashplace passes over-center in thedirection toward the position illustrated in FIG. 4, and the fixed unit20 functions as a pump, driven by the gear G4, rotating in the directionof the carrier to subtract speed in the differential.

If the output speed exceeds that desired, the flyweights 132 move thevalve stem 134 to communicate the control cylinder 60 with the drainconduit 144 to increase the displacement and subtract more speed. If theoutput speed falls below the desired value, the flyweights allow thevalve stem 134 to communicate the control cylinder 60 with the chargeport 142, thereby to reduce displacement in the variable unit 19 andsubtract less speed in the differential. Under these conditions, it willbe understood that the variable unit, being driven at a constant speed,functions as a metering device which has the effect of slowing the unit20 when the displacement is reduced, and increasing the speed of theunit 20 when the displacement is increased.

it will be understood that the system described has the advantages thatthe generator may be accelerated to its synchronous speed preparatory tothe starting operation without significant load, the engine is startedin a purely hydrostatic mode with a minimum of losses, and the driveoperates at a high efficiency with most of the power transmittedmechanically in a constant speed generating mode. Only one of thehydraulic units needs to be of the variable displacement type and thesize of the hydraulic units is minimized. The starting function isprogrammed by the magnetic trim control of the governor valve with apressure limit determined by the torque valve. The constant speedgenerating mode is controlled by the governor responsive to outputspeed.

It should be understood that while the graphs in FIGS. 2 and 3 refer tospecific speeds in which it is contemplated that the engine shaft mayattain a speed in excess of 10,000 rpm, the system is not limited tosuch values. For example, in one system the maximum speed of the engineshaft is more like 7,500 rpm. In that situation, the values in thegraphs of FIGS. 2 and 3 would be on the order of three-fourths of thatshown.

l claim:

1. A starter-Drive for use between an aircraft engine and a generator totransmit power in either direction, between the engine and thegenerator, for driving the engine from the generator in a starting mode,and for driving the generator from the engine in a generating mode,comprising,

a. a first shaft connectible with a generator,

b. a second shaft connectible with an engine,

c. a mechanical gear differential including one element rotatable withthe first shaft,

a variable ratio hydrostatic transmission including a first hydraulicunit connected for rotation with the first shaft, a second hydraulicunit connected in closed hydraulic circuit with the first hydraulic unitand connected forrotation with a second element in the differential,and-means for varying the displacement of at least one of the hydraulicunits,

e. a first one-way clutch connecting the second hydraulic unit to thesecond shaft to drive the latter exclusively through the hydrostatictransmission when the generator is operated as a motor to drive theengine, and

f. a second one-way clutch'connecting a third differential element tothe second shaft to transmit power from the engine to the generatorthrough the differential and the hydrostatic transmission after theengine is started.

2. A starter-drive as defined in claim 1, wherein the displacementvarying means is connected to vary the displacement of the firsthydraulic unit in opposite directions from neutral.

3. A starter-drive as defined in claim 2, including means controllingthe displacement varying means to vary the displacement in one directionfrom neutral during the starting mode and in the other direction fromneutral during the generating mode.

4. A starter-drive as defined in claim 1, including a the secondhydraulic unit during the starting mode and returning low pressure fluidduring the generating mode, a second conduit connecting the hydraulicunits for supplying high pressure fluid from the second hydraulic unitto the first hydraulic unit during the generating mode and returning lowpressure fluid during the starting mode, a control valve responsive topressure in the first conduit and controlling the displacement varyingmeans to limit the pressure in the first conduit, and means forestablishing a high pressure setting on the control valve during thestarting mode and a low pressure setting during the generating mode.

5. A starter-drive as defined in claim 4, including means forestablishing an intermediate pressure setting on the control valve fordriving the engine without starting.

6. A starter-drive as defined in claim 1, including a governor valveresponsive to speed of the first shaft and controlling the displacementvarying means during the generating mode to provide constant speed ofthe first shaft regardless of variation in the speed of the secondshaft.

7. A starter-drive as defined in claim 6, including selectively operablebias means controlling the governor valve during the starting mode tovary the displacement to bring the second shaft up to speed.

8. A starter-drive for transmitting power in either direction between anaircraft engine and a generator, comprising,

i a. a first shaft connectible ma generator,

b. a second shaft connectible to an engine,

c. a differential including a first gear rotatable with the first shaft,a second gear rotatable with the second shaft, and a control gear,

d. a first hydraulic unit connected for rotation with the first shaft,

e. a second hydraulic unit connected in closed hydraulic circuit withthe first hydraulic unit and connected for rotation with the controlgear,

f. means for varying the displacement of the first hydraulic unit inopposite directions from neutral, g. a first one-way clutch connectingthe second hydraulic unit to the second shaft to drive the latterexclusively through the hydraulic units when the generator is operatedas a to start the engine while allowing the second shaft to overrun thesecond hydraulic unit after the engine is started,

h. a second one-way clutch connecting the second shaft to transmit-powerfrom the second shaft to the second gear after the engine is startedwhile allowing the second gear to overrun during starting,

i. a governor valve responsive to the speed of the first shaft tocontrol the displacement varying means after the engine is started toadd or substract speed in the differential and provide constant speed ofthe first shaft despite variations in the speed of the second shaft, and

j. selectively operable biasing means controlling the governor valveduring engine starting to vary the displacement to bring the secondshaft up to speed.

9. A starter-drive as defined in claim 8, including a first conduitconnecting the hydraulic units for supplying high pressure fluid fromthe first hydraulic unit to the second hydraulic unit during enginestarting and returning low pressure fluid during generating, a secondthe displacement to limit pressure, and means establishing a highpressure setting on the control valve during starting and a low pressuresetting during generating.

1. A starter-Drive for use between an aircraft engine and a generator totransmit power in either direction, between the engine and thegenerator, for driving the engine from the generator in a starting mode,and for driving the generator from the engine in a generating mode,comprising, a. a first shaft connectible with a generator, b. a secondshaft connectible with an engine, c. a mechanical gear differentialincluding one element rotatable with the first shaft, d. a variableratio hydrostatic transmission including a first hydraulic unitconnected for rotation with the first shaft, a second hydraulic unitconnected in closed hydraulic circuit with the first hydraulic unit andconnected for rotation with a second element in the differential, andmeans for varying the displacement of at least one of the hydraulicunits, e. a first one-way clutch connecting the second hydraulic unit tothe second shaft to drive the latter exclusively through the hydrostatictransmissiOn when the generator is operated as a motor to drive theengine, and f. a second one-way clutch connecting a third differentialelement to the second shaft to transmit power from the engine to thegenerator through the differential and the hydrostatic transmissionafter the engine is started.
 2. A starter-drive as defined in claim 1,wherein the displacement varying means is connected to vary thedisplacement of the first hydraulic unit in opposite directions fromneutral.
 3. A starter-drive as defined in claim 2, including meanscontrolling the displacement varying means to vary the displacement inone direction from neutral during the starting mode and in the otherdirection from neutral during the generating mode.
 4. A starter-drive asdefined in claim 1, including a first conduit connecting the hydraulicunits for supplying high pressure fluid from the first hydraulic unit tothe second hydraulic unit during the starting mode and returning lowpressure fluid during the generating mode, a second conduit connectingthe hydraulic units for supplying high pressure fluid from the secondhydraulic unit to the first hydraulic unit during the generating modeand returning low pressure fluid during the starting mode, a controlvalve responsive to pressure in the first conduit and controlling thedisplacement varying means to limit the pressure in the first conduit,and means for establishing a high pressure setting on the control valveduring the starting mode and a low pressure setting during thegenerating mode.
 5. A starter-drive as defined in claim 4, includingmeans for establishing an intermediate pressure setting on the controlvalve for driving the engine without starting.
 6. A starter-drive asdefined in claim 1, including a governor valve responsive to speed ofthe first shaft and controlling the displacement varying means duringthe generating mode to provide constant speed of the first shaftregardless of variation in the speed of the second shaft.
 7. Astarter-drive as defined in claim 6, including selectively operable biasmeans controlling the governor valve during the starting mode to varythe displacement to bring the second shaft up to speed.
 8. Astarter-drive for transmitting power in either direction between anaircraft engine and a generator, comprising, a. a first shaftconnectible to a generator, b. a second shaft connectible to an engine,c. a differential including a first gear rotatable with the first shaft,a second gear rotatable with the second shaft, and a control gear, d. afirst hydraulic unit connected for rotation with the first shaft, e. asecond hydraulic unit connected in closed hydraulic circuit with thefirst hydraulic unit and connected for rotation with the control gear,f. means for varying the displacement of the first hydraulic unit inopposite directions from neutral, g. a first one-way clutch connectingthe second hydraulic unit to the second shaft to drive the latterexclusively through the hydraulic units when the generator is operatedas a to start the engine while allowing the second shaft to overrun thesecond hydraulic unit after the engine is started, h. a second one-wayclutch connecting the second shaft to transmit power from the secondshaft to the second gear after the engine is started while allowing thesecond gear to overrun during starting, i. a governor valve responsiveto the speed of the first shaft to control the displacement varyingmeans after the engine is started to add or substract speed in thedifferential and provide constant speed of the first shaft despitevariations in the speed of the second shaft, and j. selectively operablebiasing means controlling the governor valve during engine starting tovary the displacement to bring the second shaft up to speed.
 9. Astarter-drive as defined in claim 8, including a first conduitconnecting the hydraulic units for supplying high pressure fluid fromthe first hydraulic unit to The second hydraulic unit during enginestarting and returning low pressure fluid during generating, a secondconduit connecting the hydraulic units for supplying high pressure fluidfrom the second hydraulic unit to the first hydraulic unit duringgenerating and returning low pressure fluid during starting, a controlvalve responsive to pressure in the first conduit and controlling thedisplacement to limit pressure, and means establishing a high pressuresetting on the control valve during starting and a low pressure settingduring generating.